1. Field of the Invention
The present invention relates to print ribbon delivery systems for printers, and more particularly, to a system for detection of motion of the ribbon in a ribbon cartridge.
2. Description of the Prior Art
Detection of ribbon motion is important for at least three reasons: 1) If the ribbon has stalled, the ink in the ribbon will be rapidly depleted and print information will be lost, 2) A ribbon stall condition may be an indicator of a mechanical failure of the ribbon cartridge or transport system, indicting that a repair is needed, or 3) The ribbon has become hung in the wires of the print head, in the case of an impact printer, and that the print head may already be, or may become, damaged.
Previous means for sensing ribbon motion include optically sensing the movement of a hole in the ribbon fabric and optically sensing the motion of a flag attached to an idler roller in the ribbon path (see U.S. Pat. No. 4,988,224 by Furrow and Johenning). These means for sensing ribbon motion are undesirable because of 1) a hole requires a modification to the ribbon fabric increasing fabric cost 2) sensing a hole in the ribbon fabric can introduce long delays in detecting failed ribbon motion which can cause loss of print data and or damage to the print head/print mechanism 3) hole/s in the ribbon fabric can reduce printer print speed performance since it is not desirable to print in the hole. Printing must be suspended for a period of time to allow the hole to pass the print zone before printing can resume, and 4) Optical sensors can malfunction due to contamination such as paper and ribbon dust that are present in impact printers.